Liquid crystal display devices (LCD) are much more advantageous than CRT (Cathode Ray Tubes) because of their small thickness, light weight and low power consumption. The liquid crystal display device comprises a liquid crystal cell and a pair of polarizing elements disposed on both sides of the cell respcecively. The liquid cell comprises rod-like liquid crystalline molecules, two substrates for encapsulating them, and electrodes for applying a voltage to the rod-like liquid crystalline molecules. In order to orient the encapsulated rod-like liquid crystalline molecules, the two substrates have an orientation membrane.
In order to remove coloration of an image displayed on the liquid crystal cell, an optical compensatory sheet (retardation plate) is often provided between the liquid crystal cell and the polarizing element. A laminate of the polarizing element (polarizing membrane) and the optical compensatory sheet functions as an elliptically polarizing plate. In some cases, a function of enlarging the viewing angle of the liquid crystal cell is imparted to the optical compensatory sheet. As the optical compensatory sheet, stretched birefringent films have conventionally been used.
It has also been proposed to use an optical compensatory sheet comprising a transparent support having provided thereon an optically anisotropic layer containing a discotic compound in place of the stretched birefringent films. The optically anisotropic layer is formed by orienting the discotic compound and fixing the oriented alignment. The discotic compounds generally have a large birefringent index. Also, discotic compounds give various orientation forms. Therefore, by using such discotic compounds, there can be prepared an optical compensatory sheet having optical properties not obtainable by the conventional stretched birefringent films. As to the optical compensatory sheets using the discotic compounds, descriptions are given in the specifications of JP-A-6-214116, U.S. Pat. Nos. 5,583,679 and 5,646,703, and West German Patent 3911620A1.
U.S. Pat. Nos. 4,583,825 and 5,410,422 disclose a liquid crystal display device using a bend orientation mode liquid crystal cell wherein rod-like liquid crystalline molecules located in the upper portion of the liquid crystal cell and those located in the lower portion of the liquid crystal cell are oriented substantially in reverse directions (symmetrically). Since the rod-like liquid crystalline molecules in the upper portion and those in the lower portion are symmetrically oriented with each other, the bend orientation mode liquid crystal cell has a function of self optical compensation. Therefore, this liquid crystal mode is also called OCB (Optically Compensatory Bend) liquid crystal mode. The bend orientation mode liquid crystal display device has the advantage of a high response speed.
In comparison with the general liquid crystal modes (TN mode, STN mode), the bend orientation mode has the advantage of a wider viewing angle and a faster response speed. In comparison with CRT, however, it must be more improved. In order to more improve the bend orientation mode liquid crystal display device, it may be considered to use an optical compensatory sheet similarly to the general liquid modes. However, an optical compensatory sheet composed of a conventional stretched birefringent film shows only an insufficient optically compensating function in the bend orientation mode liquid crystal display device.
As has been described hereinbefore, it has been proposed to use, in place of the stretched birefringent film, an optical compensatory sheet having an optically anisotropic layer containing a discotic compound and a transparent support. JP-A-9-197397 (U.S. Pat. No. 5,805,253) and International Patent Publication WO96/37804 (European Patent 0783128A) disclose a bend orientation mode liquid crystal display device using an optical compensatory sheet containing the discotic compound. By using the optical compensatory sheet containing the discotic compound, a viewing angle of the bend orientation mode liquid crystal display device is markedly improved.
Further, JP-A-11-316378 describes that examination of the use of an optical compensatory sheet containing a discotic compound in the bend orientation mode liquid crystal display device revealed the problem that a light of a particular wavelength is leaked to cause coloration on a displayed image. It describes that the cause of this coloration lies in wavelength dependency of the transmission of an elliptically polarizing plate (a laminate of the polarizing element and the optical compensatory sheet).
And, in this publication, an excellent optical compensation effect is obtained by disposing an optically anisotropic layer and a polarizing membrane so that the angle between the average direction of orthogonal projection of the normal lines of disc plane of the discotic compound to the transparent support plane and the in-plane transparent axis of the polarizing membrane becomes substantially 45°.
On the other hand, a liquid crystal display device using a low-temperature polysilicon TFT has been known to permit high-speed charging and realize an increased opening ratio.
However, in the case where such low-temperature polysilicon TFT is used in the above-mentioned bend orientation mode liquid crystal cell, the conventional optical compensation film fails to attain a sufficient improvement of a viewing angle when it is intended to improve the viewing angle by the optical compensatory film. Thus, there has been a demand for a liquid crystal display device which can meat requirements for low electric power to consume, high response speed and wide viewing angle.